The present invention relates to a two-piece golf ball including a core made from a rubber-like elastic body and a cover formed to cover the core, and particularly to a two-piece golf ball capable of increasing the flying distance of the ball without impairing both the feeling of hitting the ball and the durability of the ball.
In general, two-piece golf balls, which are capable of increasing the flying distance of the ball and prolonging the durability of the ball as compared with thread-wound type golf balls, have been widely used for golfers.
A two-piece golf ball, however, is different from a thread-wound type golf ball having been accustomed for a long time by golfers, in terms of feeling of hitting the ball. Accordingly, the technique of improving a two-piece golf ball from the viewpoint of feeling of hitting the ball has been successively examined.
For example, Japanese Patent No. 3040300 has proposed a two-piece golf ball, wherein the feeling of hitting the ball is improved to a level comparable to that of a thread-wound type golf ball.
On the other hand, the golfer""s demand to increase the flying distance of a golf ball has been strong yet, and therefore, although two-piece golf balls are capable of increasing the flying distance of the ball as compared with thread-wound type golf balls, it is expected to develop a two-piece golf ball capable of further increasing the flying distance of the ball.
Various kinds of two-piece golf balls improved in feeling of hitting the ball have been proposed, for example, in the above-described document, Japanese Patent No. 3040300; however, these two-piece golf balls each fail to take a good balance against other performances as described below.
For example, with respect to the two-piece golf ball proposed in Japanese Patent No. 3040300, the feeling of hitting the ball becomes softer than that of a conventional ball; however, the cover is not particularly specified except for the thickness of the cover, which is described to be preferably in a range of 1.4 mm to 2.7 mm and to be set to 1.6 mm in each of the disclosed embodiments. With such a thickness of the cover, it is difficult to improve both the durability and the flying performance of the ball, and particularly, in consideration of a difference in JIS-C hardness between the core surface and the core center, which is described to be preferably in a range of 28 to 34, it becomes probably very insufficient to improve the durability against cracking of the ball as well as the flying performance of the ball, and further there is a problem associated with a composition of a material for obtaining such a core.
In view of the foregoing, the present invention has been made, and an object of the present invention is to provide a two-piece golf ball capable of increasing the flying distance of the ball without impairing both the feeling of hitting the ball and the durability of the ball.
To achieve the above object, the present inventor has earnestly studied a two-piece golf ball having a core and a cover on a difference in JIS-C hardness between the core surface and the core center and the compressive deformation amount of the core in a specified range, and on the thickness of the cover formed to cover the core.
The present inventor has eventually found that a two-piece golf ball including a core and a cover, which is characterized in the following points (1) to (4), is surprisingly excellent in either of the flying performance of the ball, the feeling of hitting the ball, and the durability of the ball, and that the golf ball particularly exhibits a flying performance sufficiently acceptable to golfers at the level of low/intermediate head speeds (about 35 to 40 m/s).
(1) The core is made from a rubber composition containing a base rubber to which an organic sulfur compound is mixed in an amount of 0.05 part by mass or more on the basis of 100 parts by mass of said base rubber;
(2) the JIS-C hardness, measured by a JIS-C hardness tester, of the center of said core is in a range of 40 to 60 and the JIS-C hardness of the surface of said core is in a range of 70 to 85, and further, a difference in JIS-C hardness between the core surface and the core center (hardness of core surfacexe2x88x92hardness of core center) is in a range of 20 to 27;
(3) the compressive deformation amount, which is measured by applying, to said core, a load from an initial load of 10 kgf (98.07 N) to a final load of 130 kgf (1274.91 N), is in a range of 3.5 to 4.6 mm; and
(4) the thickness of said cover is in a range of 1.8 to 2.3 mm.
Accordingly, the present invention provides a two-piece golf ball including a core and a cover, wherein said core is made from a rubber composition containing a base rubber to which an organic sulfur compound is mixed in an amount of 0.05 part by mass or more on the basis of 100 parts by mass of said base rubber; the JIS-C hardness, measured by a JIS-C hardness tester, of the center of said core is in a range of 40 to 60 and the JIS-C hardness of the surface of said core is in a range of 70 to 85, and further, a difference in JIS-C hardness between the core surface and the core center (hardness of core surfacexe2x88x92hardness of core center) is in a range of 20 to 27; the compressive deformation amount, which is measured by applying, to said core, a load from an initial load of 10 kgf (98.07 N) to a final load of 130 kgf (1274.91 N), is in a range of 3.5 to 4.6 mm; and the thickness of said cover is in a range of 1.8 to 2.3 mm.
In this two-piece golf ball, preferably, the cover is made from a material containing a main resin to which an inorganic granular filler is mixed as a reinforcing material in an amount of 11 parts by mass or more on the basis of 100 parts by mass of said main resin.
With these configurations of the two-piece golf ball, it is possible to further increase the flying distance of the ball without impairing the feeling of hitting the ball and the durability of the ball.